Field of the Disclosure
The present disclosure relates to an optical element for use in an optical apparatus such as a camera, a manufacturing method thereof, and an optical apparatus including the optical element.
Description of the Related Art
An optical element, such as a lens or a prism, for use in an optical system of an optical apparatus, such as a camera or a projector, uses therein an antireflection portion for preventing unwanted light other than focused light flux from being incident upon the image plane. Typical examples of such unwanted light include reflected light from a light input/output surface of the optical element, and reflected light from the circumferential edge portion of an end face of the optical element. These types of unwanted light may cause flare and/or ghost. To avoid such unwanted light, the following techniques have conventionally been used.
Technique 1: An antireflection portion for increasing the transmittance of incident light to reduce the reflectance is formed in the light active portion, thereby preventing unwanted light from being generated.
Technique 2: An antireflection portion for increasing the absorptance of light to reduce the reflectance is formed in the circumferential edge portion, which is a light inactive portion, thereby preventing unwanted light from being generated.
A common conventional method for Technique 1 is to form an antireflection film that is a multilayer stack of dielectric thin films, generally called “multicoat.” Another known method is to form a film of boehmite, which is a hydroxyoxide of aluminum, on a substrate to provide an antireflection effect. In the latter case, an antireflection film is formed as follows. A film is formed using a vacuum deposition method or using a liquid phase method (sol-gel method). Next, drying or baking is performed to obtain an aluminum oxide film, which is then treated by water vapor, or immersed in warm water, to transform the surface layer into a boehmite film. Thus, an irregularity structure having a dimension at the lower limit of, or below, the used wavelength range is formed to produce an antireflection film.
A common conventional method for Technique 2 is to form a light shielding film in the circumferential edge portion. To improve the light shielding capability by scattering and to improve the adhesion property of the light shielding film, the circumferential edge portion is formed to be a rough surface having an arithmetic mean roughness Ra in a range from about 1 μm to about 50 μm. In addition, because of the practical difficulty of completely aligning an end of the light shielding film formed on the rough surface with an end of the rough surface, the light shielding film is formed to allow the edge portion thereof to partly overlap a smooth surface.
If a gap covered with no film exists between the antireflection film and the light shielding film in a boundary portion between the light active portion of Technique 1 and the light inactive portion of Technique 2, strong unwanted light may be generated. To avoid such situation, the antireflection film and the light shielding film are formed to partly overlap each other. Considering a fact that a smaller refractive index difference between the antireflection film and the substrate of the optical element can reduce reflection at a higher degree, the antireflection film and the light shielding film are generally formed such that the antireflection film is formed on the substrate, and the light shielding film is formed on the antireflection film.
In recent years, for improving performance of an antireflection film, a method has been increasingly used in which, rather than the multicoat antireflection film described above, an antireflection film having an irregularity structure having a dimension at the lower limit of, or below, the used wavelength range is formed. Japanese Patent Laid-Open No. 2015-176016 discloses a configuration including an antireflection film having an irregularity structure, as an antireflection portion for a light active portion, on a surface of the antireflection film, and a light shielding film formed in a light inactive portion around the light active portion, wherein an edge portion of the light shielding film extends to the light active portion to overlap an edge portion of the antireflection film.
If, as described above, an optical element is configured such that an edge portion of the antireflection film having an irregularity structure and an edge portion of the light shielding film overlap each other, and this overlapping portion resides over the smooth surface, film blister and/or film peeling (hereinafter referred to collectively as “film blister/peeling”) is likely to occur at the interface between the antireflection film and the substrate. This presents a problem in that ghost and/or flare are caused in an optical apparatus including such optical element. Ghost and flare readily occur particularly in a harsh environment, such as a high temperature, a low temperature, and a high humidity, and may thus occur by aging during usage. Such blister/peeling of an antireflection film can be reduced or eliminated generally by improving the adhesion between the substrate and the antireflection film. However, an improvement in the adhesion between the substrate and the antireflection film will require a certain solution, such as changing the material of, or adding another material to, the antireflection film, and/or disposing an underlying layer formed of another material over the substrate. Such solutions are unfavorable in view of the adverse effect on the antireflection performance, and in view of an increase in the material cost and in the number of manufacturing steps.